Fix a problem with cursors, which did not happen with unbuffered PS for

Fix a problem with cursors, which did not happen with unbuffered PS for
some reason. Double free of the data, which led to valgrind warnigns.
The fix actually optimizes the code in this cases because the old code
used copy_ctor while the new one skips it because it is not needed.
Transferring data ownership and nulling works best, for PS where we
always copy the string from the result set, unlike the text protocol.